Mathematical calculation to determine focal reducer focus

[Dennis G]

Is there someone out there who has the knowledge to calculate the focal point of a .5 and .7 focal reducer?
Added to my Toucam I cannot quite get the image focused and rather than pull the scope down, using trial and error methods, I'd like to be able to measure the known focal point before making any drastic changes.
Dennis G

[iceman (Mike)]

What's the focal length of your scope?

Can you post a picture of the ToUcam in the scope?

I can get my ToUcam to focus in my 10" dob (now sold) at prime focus (no barlows) as well as with any size barlow (2x through to 5x).

[Geoff45 (Geoff)]

Offhand I don't think this can be done unless you know more about the various parameters in the system. Different designs of the focal reducer can put the image at a different distance behind the focal reducer lens and still get the same focal reduction.
Geoff

[avandonk]

When you insert a focal reducer into a system you always need to go in to attain focus.
The formulae are
Focal reduction = (F2 - x)/F2
That is in words
Focal reduction = (focal length of FR - distance from sensor)/(focal length of FR)

D= F1- (F2 X x)/(F2-x) Note that X is multiply by

Where F1 is the focal length of the telescope optic
F2 focal length of focal reducer
x distance between focal reducer and sensor
D distance of focal reducer from telescope optic

And focus plane shift in is = F1-(x+D)

all very simple hey?

Do you want a diagram?

Bert